The present invention relates to optical switching devices and in particular to a thermally responsive optical switching device which provides over-temperature protection to solar thermal collectors.
Rising fuel costs and the general desirability of energy independence has bolstered the development of solar energy alternatives to typical fossil fuel home heating. Solar thermal energy converters have been proposed as viable alternatives for petrochemical and similarly conventional home heating means. Manufacturers of lower temperature home heating collectors have continually sought lower cost alternatives to improve the economic effectiveness of solar energy. An approach taken by a number of solar panel manufacturers is to substitute plastic components for the more costly copper and other metallic components. See, for example, U.S. Pat. Nos. 3,918,430 and 4,046,135. Although plastic components may readily be substituted for many typical collector parts, these plastic components conventionally have upper working temperature limits in the general operating temperature range of the collector. These plastic components function successfully under normal collector system operation. However, in the instance where the collector's heat transfer means is interrupted, the temperature of the collector may rise substantially. The temperature in a conventional flat plate collector in the absence of interruption of heat exchange means may exceed 150.degree. C. A limited number of plastics can withstand these temperatures and plastics capable of withstanding these temperatures are generally more costly than their metal counterparts.
Numerous devices have been proposed to provide over-temperature protection for solar collector devices. These devices may be generally categorized as either mechanical or optical means for inhibiting light from reaching the absorber surface. Prior teachings have been directed principally toward mechanical devices for reducing the amount of light reaching the solar absorbing surface. Examples of such devices may be found in U.S. Pat. No. 3,822,692 and U.S. Pat. No. 3,964,464. Such devices add to the cost, complexity and weight of the solar panel making the panel less desirable both from a technical and economic viewpoint.